1. Field of the Invention
The present invention relates to a Voice over Internet Protocol (VoIP) service system using Network Address Translation (NAT) and a method of processing a packet in the system, and more particularly, to a VoIP service system normally transferring a packet from a terminal located outside a NAT device to a terminal located behind the NAT device using an IP conntrack generated to transfer a packet for VoIP service between a terminal located outside the NAT device and a terminal located behind the NAT device, and a method of processing a packet in the system.
2. Description of the Related Art
VoIP refers to Internet telephone technology for a series of devices that transfer voice information using an IP. VoIP is not based on a line, as a Public Switched Telephone Network (PSTN) is, but transfers voice information in discontinuous packets in digital form.
Internet telephone technology's main advantage is that it implements integrated voice communication service using a conventional IP network as is, and thereby allows telephone users to receive long distance and international call services in the Internet and intranet environments at a local call rate.
Signaling protocols for implementing VoIP are H.323, a Session Initiation Protocol (SIP), a Media Gateway Control Protocol (MGCP), and so on. Currently, the SIP that is an application layer control protocol based on a simple text has been widely commercialized.
Meanwhile, NAT is defined in Requests For Comments (RFC) 3022 as a translation technique that translates a private address into a public address or vice versa. Such a NAT device, e.g., a router, makes an internal zone behind the NAT device using a private address and an external zone outside the NAT device using a public address correspond to each other. And, the NAT technique does not externally expose a network behind the NAT device, thus having a security effect also.
Such a NAT technique allows terminals that are located in private networks due to the exhaustion of Internet Protocol version 4 (IPv4) addresses, to access the Internet.
In addition, the NAT device, e.g., a router, changes a port number using a Network Address Ports Translation (NAPT) table so that a plurality of terminals can communicate with each other using one public address, thereby distinguishing communication performed between the terminals.
The development of network technology is leading to the development of a gateway that transfers voice information between terminals located in private networks and terminals connected with the Internet.
Such a gateway must have a NAT or NAPT function and a VoIP service function to transfer voice information between terminals located in private networks and terminals connected with the Internet.
A gateway must have a NAT table to provide VoIP service between terminals located in a private network and terminals connected with the Internet. Here, NAT configuration may be made by an administrator, or automatically made when the gateway is installed. The former is a manual configuration method, and the latter is an automatic configuration method.
Meanwhile, when a packet is transferred from a terminal connected with the Internet to a terminal located in a private network, a gateway generates an IP conntrack using information included in the transferred packet to transfer the packet from the terminal located in a private network to the terminal connected with the Internet. It should be noted here that the known phrase IP conntrack is associated with connection tracking and is not equivalent to the known phrase IP address.